When surgeons remove (resect) a diseased segment of the gastrointestinal tract, its continuity is reestablished by carefully suturing or stapling th remaining ends together to create a viable and well- sealed connection referred to as an anastomosis. The most devastating and disability complication following anastomotic construction is a leak where anastomotic tissues fail to heal, separate, and intestinal contents escape into the peritoneum causing sepsis, peritonitis and even death. Anastomotic leak rates have not changed in decades and remain at 10% for the highest risk cases involving the esophagus and rectum. We have generated compelling evidence that intestinal microbes are the driving force behind the pathogenesis of anastomotic leak. Our work demonstrates that certain intestinal microbes become activated in vivo to produce a high degree of collagenase that is capable of expressing a tissue destroying phenotype leading to anastomotic non- healing, separation, and leak. We show that intestinal microbes become transformed in vivo to express high collagenase over the course of recovery that can cleave intestinal MMP9 to its active form and breakdown the collagen needed for healing. Importantly our data demonstrate that currently used IV antibiotic regimens do not eliminate these offending microbes whereas direct application of topical antibiotics (DTA) to anastomotic tissues completely protects against anastomotic leak by suppressing collagenase and preventing MMP9 cleavage to its active form. We have generated preliminary data in humans that strongly suggests that patients harboring pathogens expressing the leak phenotype are most at risk when their normal indigenous microbiota can no longer constrain the tissue damaging effects of these renegade pathogens. In this planning proposal we will develop a trial to directly test this hypothesis. We will plan a tril in which patients undergoing a low colo- rectal resection, termed a low anterior resection (LAR) with a diverting protective stoma (ileostomy), are subjected to serial endoscopic examination (SES) by sigmoidoscopy with image capture and microbial analysis at 3 separate intervals following surgery. Images of anastomotic healing will be correlated to compositional and functional changes in the microbiota via 16s rRNA sequencing and phenotype assay including collagenase and MMP9 cleavage activity. In the SES-DTA trial, patients will be randomized to application of direct topical antibiotics (DTA) versus placebo and anastomotic healing tracked at 3 SES episodes over the course of 21 days (postoperative days 2-3, 7-10, and 14-21). This planning proposal has assembled uniquely qualified investigators who are leading authorities on intestinal microbiome research, the pathogenesis of anastomotic leak colorectal surgery, multi-centered trials on patients undergoing colorectal surgery, and all facets of biostastistics and bioinformatics to operationalize the trial. This trial will be the first of its kind to directly exmine anastomotic tissues as they heal and correlate healing to microbial composition and function. It will change the practice of colo-rectal surgery by providing a clinical path to predict and prevent anastomotic leak.